The red mud problem in Aluminium production
The mineral used to make Alumiunium is called bauxite. It contains alumina minerals and alumino-silicate clays but also many other gangue (tramp) elements, some of which are very rare and precipus while others are highly toxic. The insoluble parts of the bauxite are removed by exposing the bauxite ore with very hot caustic soda (chemcially: sodium hydroxide) in the Bayer process.
The Bayer process produces not only the highly desired Al oxide from the bauxite ore, but it also produces an undesired by-product, called red mud. It contains toxic heavy metals and its high alkalinity makes it extremely corrosive and damaging to soil and life forms, creating a huge challenge for such disposals. This debris which is both valaabel and toxic is one the biggest problems in Aluminium making.
The annual production of smelter and chemical grade alumina is higher than 130 million tons per year, mostly produced by the Bayer process.
The global average of bauxite residue generated per tonne of alumina is between 1 and 1.5 tonnes.
This means that more than 150 million tons of bauxite residue are produced every year, with more than 6 million tons in Europe alone.
The majority of this highly toxic (and in part also valuable) waste is deposited as landfill.
Worldwide more than 100 Bayer plants are operative.
The total stockpile of this bauxite residue exceeds 3500 million tons.
This rapidly and continuously increasing production of red mud as a by-product every year is an imminent threat to people, plants and the entire environment.
What is the chemical composition of red mud?
Basically, red mud contains all the gangue substances that are already contained in the original bauxite ore that is the basis of winning Al oxide.
These are mainly iron and titanium oxides and various silicic acid compounds.
Red mud obtains its characteristic red colour due to its main component, iron(III) oxide, which could be even a very valuable future ressource for green steel making.
Minor constituents contained to a lesser extent in bauxite ore vary with the origin of the material. Several toxic and heavy metals such as arsenic, lead, cadmium, chromium, vanadium or mercury occur in bauxite and thus also in the red mud .
The dry mass of had red mud can have a content of 110 ppm arsenic, 1.3 ppm mercury as well as 660 ppm chromium.
More detailed measurements show that the red mud can have more than 40 chemical elements, some with a fraction of several 100s of ppm, including cadmium, nickel, arsenic and antimony.
Composition vary significantly with the origin of the ore.
Opportunities if red mud processing
Besides these multiple threats associated with the red mud's toxic element content there are also substantial business and sustainability opportunities in this field, as red mud contains many valuable ressources too.
In average red mud contains up to 65-70% iron oxide (Fe2O3), 40-50% aluminum oxide (Al2O3), and 15-25% titanium dioxide (TiO2), depending on origin of the bauxite ore.
Of specific interest is the iron contained in the form of oxides and hydroxides, hence its intense red color. It can be used in the sintering process of alumina production, for iron making, and in pigment applications.
Beyond the Bayer process where the Al oxide is won, there still remains a very high amount of aluminum components in the red mud, hence, efforts to win the remaining aluminum from red mud is a most attractive target.
Titanium exists in the red mud as titanium dioxide (TiO2). It is used in a number of products as pigment, even offering protection from UV rays, which has lent it to use in cosmetics and sunscreen products. It may also be used in more industrial applications as a pigment.
Of very interest are the so called rare earth elements - these are urgently needed in a wide array of applications, most notably in magnets, alloy design, electronics, with massive consumption in so called high and green technologies, where they are often irreplaceable in many products.
Scandium is also an element which can be extracted from the red mud.
In addition to these metals, red mud contains also Silicon Dioxide (SiO2), Calcium Oxide (CaO), Sodium Oxide (Na2O), Potassium Oxide (K2O)
Lithium Oxide (Li2O), Vanadium Pentoxide (V2O5), and Zirconium Dioxide (ZrO2), all highly valuable ressources. Today, practically all oif these are just dumped as toxic landfill - lots of
room for basic metallurgical real research ahead!